Retrograde optogenetic characterization of the pontospinal module of the locus coeruleus with a canine adenoviral vector.

Noradrenergic neurons of the brainstem extend projections throughout the neuraxis to modulate a wide range of processes including attention, arousal, autonomic control and sensory processing. A spinal projection from the locus coeruleus (LC) is thought to regulate nociceptive processing. To characterize and selectively manipulate the pontospinal noradrenergic neurons in rats, we implemented a retrograde targeting strategy using a canine adenoviral vector to express channelrhodopsin2 (CAV2-PRS-ChR2-mCherry). LC microinjection of CAV2-PRS-ChR2-mCherry produced selective, stable, transduction of noradrenergic neurons allowing reliable opto-activation in vitro. The ChR2-transduced LC neurons were opto-identifiable in vivo and functional control was demonstrated for >6 months by evoked sleep-wake transitions. Spinal injection of CAV2-PRS-ChR2-mCherry retrogradely transduced pontine noradrenergic neurons, predominantly in the LC but also in A5 and A7. A pontospinal LC (ps:LC) module was identifiable, with somata located more ventrally within the nucleus and with a discrete subset of projection targets. These ps:LC neurons had distinct electrophysiological properties with shorter action potentials and smaller afterhyperpolarizations compared to neurons located in the core of the LC. In vivo recordings of ps:LC neurons showed a lower spontaneous firing frequency than those in the core and they were all excited by noxious stimuli. Using this CAV2-based approach we have demonstrated the ability to retrogradely target, characterise and optogenetically manipulate a central noradrenergic circuit and show that the ps:LC module forms a discrete unit. This article is part of a Special Issue entitled SI: Noradrenergic System.

Optoactivation of locus ceruleus neurons evokes bidirectional changes in thermal nociception in rats.

Pontospinal noradrenergic neurons are thought to form part of a descending endogenous analgesic system that exerts inhibitory influences on spinal nociception. Using optogenetic targeting, we tested the hypothesis that excitation of the locus ceruleus (LC) is antinociceptive. We transduced rat LC neurons by direct injection of a lentiviral vector expressing channelrhodopsin2 under the control of the PRS promoter. Subsequent optoactivation of the LC evoked repeatable, robust, antinociceptive (+4.7°C ± 1.0, p < 0.0001) or pronociceptive (-4.4°C ± 0.7, p < 0.0001) changes in hindpaw thermal withdrawal thresholds. Post hoc anatomical characterization of the distribution of transduced somata referenced against the position of the optical fiber and subsequent further functional analysis showed that antinociceptive actions were evoked from a distinct, ventral subpopulation of LC neurons. Therefore, the LC is capable of exerting potent, discrete, bidirectional influences on thermal nociception that are produced by specific subpopulations of noradrenergic neurons. This reflects an underlying functional heterogeneity of the influence of the LC on the processing of nociceptive information.

Sensory activation and role of inhibitory reticulospinal neurons that stop swimming in hatchling frog tadpoles.

Activity in neuronal networks underlying locomotion and other rhythmic actions can start and stop in response to specific sensory stimuli. In vertebrate locomotion, some reticulospinal neurons such as Mauthner neurons can initiate activity, but the neurons controlling stopping are not defined. We have studied swimming in tadpoles of the frog, Xenopus, which is started by touching the skin and stops when the head contacts a solid surface. Using an immobilized tadpole preparation, the same stimuli control fictive swimming. When head contact is imitated by pressure to the head skin sensory neurons in the trigeminal ganglion are active, spinal neurons receive GABAergic inhibition, and swimming stops. Here we record intracellularly from neurons in the hindbrain that are excited by pressure or electrical stimulation to the head skin. By intracellular filling with neurobiotin, we identify these anatomically as mid-hindbrain reticulospinal neurons (MHRs). These have contralateral descending projections to the spinal cord and GABA-like immunoreactivity. They are rhythmically inhibited during fictive swimming. Individual MHRs reliably stopped ongoing swimming when brief firing was induced by intracellular current injection. The ability of individual MHRs to stop swimming was blocked by the GABA(A) antagonist bicuculline. Our evidence indicates that MHRs receive direct excitation from trigeminal sensory neurons and in turn release GABA to directly inhibit spinal neurons and turn off the swimming central pattern generator.